1. Field of the Invention
The present invention relates to a Schottky diode which is suitable for high-voltage applications and additionally has a low forward voltage, a low leakage current, low switching losses and great robustness.
2. Description of the Related Art
High-voltage PN diodes are generally used for high-voltage applications. Such high-voltage PN diodes advantageously have a low leakage current and great robustness. Disadvantages of such high-voltage PN diodes include their high forward voltage and high switching power loss.
In such a high-voltage PN diode, the voltage is taken over mainly by the weakly doped region provided with such diodes. Electrons and holes are injected into the weakly doped region in the case of operation in the forward direction. At a high current density, high injection prevails in the weakly doped region, and the electron density and hole density are higher than the dopant concentration of the weakly doped region. The conductivity of the weakly doped region is thereby increased. This advantageously results in a reduction in the forward voltage. However, the current of a high-voltage PN diode begins to flow at room temperature only above a forward voltage of approximately UF=0.7 V. Under normal operating conditions, for example, at a current density greater than 100 A/cm2, forward voltage UF increases to values greater than 1 V. This is associated with a correspondingly high undesirable power loss. Since a high-voltage PN diode requires a thick, weakly doped region, the voltage drop in the forward direction over the weakly doped region is relatively great despite the conductivity modulation.
The charge carriers (electrons and holes) which are injected into the weakly doped region during operation in the forward direction and stored there must first be reduced during shutdown, for example, in an abrupt current commutation, before the high-voltage PN diode is at all capable of taking over the reverse voltage again. Therefore, in an abrupt current commutation, current continues to flow first in the reverse direction until the stored charge carriers have been drained off or reduced. This process, i.e., the level and duration of the drain current for reduction of the stored charge carriers, is determined primarily by the quantity of charge carriers stored in the weakly doped region. A higher and longer-lasting drain current means a higher shutdown power loss.
An improvement in the switching behavior is offered by Schottky diodes (metal semiconductor contacts and silicide semiconductor contacts). In the case of Schottky diodes, there is no high injection during forward operation and therefore the drain-off of the minority charge carriers during shutdown is eliminated. They switch rapidly and with almost no power loss. However, they are associated with high leakage currents, in particular at high temperatures and with a great voltage dependence because of the barrier-lowering effect. Furthermore, thick semiconductor layers with a low level of doping are again required for high barrier voltages, which results in unacceptable, high forward voltages at high currents. Therefore power Schottky diodes in silicon technology are not suitable for barrier voltages of more than approximately 100 V—despite the good switching behavior.
German patent DE 197 40 195 C2 describes a Schottky diode, hereinafter also referred to as a cool SBD. A significant reduction in resistance is possible with this cool SBD due to the introduction of doped p- and n-conducting columns situated alternately below a Schottky contact. If the column width is reduced, the column doping may be increased. The doping of the p and n columns is selected in such a way that when reverse voltage is applied, all doping atoms are ionized. This principle is also known as the super junction principle (SJ). Since high injection occurs during the forward operation of a high current density in a cool SBD, the ideal switching behavior of a pure Schottky diode is not achieved, but is significantly improved in comparison with a PN diode. However, the low forward voltage of a PN diode is not achieved at high currents.
FIG. 1 shows one example of such a known cool SBD. This cool SBD has an n+ substrate 10 on which an n-epitaxial layer 20 of thickness D_epi and of doping concentration ND is situated. N-epitaxial layer 20 contains etched trenches 30, which are filled with p-doped silicon of doping concentration NA and with p+-doped silicon in upper regions 40. The width of the n-epitaxial layer between adjacent trenches 30 is Wn, that of trenches 30 is Wp. Dopings and widths are selected in such a way that these regions are depleted when the full reverse voltage is applied (super junction principle). This is the case at approximately NA·Wp=ND·Wn=1012 cm−2. The n-doped regions 20 and p+-doped regions 40 are covered with a continuous metal layer 50 on front side V of the cool SBD, which is preferably implemented as a chip, this metal layer forming a Schottky contact with n-doped regions 20 and an ohmic contact with p+-doped regions 40. Metal layer 50 is the anode electrode of the cool SBD. The height of the barrier of Schottky diode 50-20 may be adjusted through the choice of a corresponding metal 50. For example, nickel or NiSi may be used as metal layer 50. If necessary, other metal layers (not shown) may be situated over functional layer 50 to make the surface solderable or bondable, for example. A metal layer or a metal system 60 which forms the ohmic contact with highly n+-doped substrate 10 is likewise situated on rear side R of the chip. This layer or layer sequence is usually suitable for soldering or other assembly. For example, it may have a sequence of Cr/NiV and Ag. Metal system 60 forms the cathode connection of the cool SBD.
The configuration described above may be regarded as a parallel circuit of Schottky diodes and PN diodes. Metal contact 50 forms Schottky diodes with n-doped columns 20. The PN structure is formed by the layer sequence of p+ region 40, p region 30 and substrate 10 as a p+/p/n+ structure.
When a reverse voltage is applied, the p- and n-doped columns are depleted. With a decline in width Wp and Wn, the doping may be increased—at least up to a certain limit, which results from the fact that the space-charge regions are already colliding at a low voltage. This reduces the path resistance of Schottky diodes 50-20-10 in the forward direction. The forward voltages are therefore lower than with a simple Schottky diode, which must be designed with a lower doping at the same reverse voltage. In addition, some current still flows through the PN diodes in the forward direction. Therefore, the forward voltage is further reduced, in particular at a high current density. However, the minority charge carriers must also be drained off again during a shutdown with negative effects for the switching time.